(1) Field of the Invention
The present invention relates to a process for producing a monofilament having a high tenacity from a thermoplastic resin, such as polyethylene, polypropylene, polyamide, polyester and the like, by a melt spinning and stretching technique.
(2) Description of the Prior Art
Heretofore, monofilaments obtained from a melt spinning and stretching of thermoplastic resins have been generally produced as follows. For instance, the thermoplastic resin is extruded through nozzles each having a round cross-sectional area, and usually passes through a cooling bath, or is optionally solidified by using a treatment bath to form fibrous materials. The fibrous materials are then stretched or drawn at a low stretching ratio of, for example, 3 through 10 and at an optimum temperature depending upon the type of resin used. Thus, monofilaments having a straight strength of 2 g/d through 7 g/d are produced. In order to increase the tenacity of the monofilaments, the use of a higher stretching ratio of, for example, 11 through 20 is required. However, in this case, although the straight strength is increased, the knot strength is remarkably decreased with the increase in the stretching ratio. Furthermore, in order to increase the stretching ratio, unstretched filaments having a higher denier should be used and, as a result, vacuum bubbles are generated in the filaments due to the deviation of the heat shrinkage at the cooling step in the inner portions of the filaments. The bubbles cause frequent stretching failure. In addition, in the case where filaments are stretched at a higher stretching ratio, other problems including the whitening of the filaments, the generation of fluff and powdering on the surface of the filaments and the like, occur.
Especially, monofilaments made of polyethylene are widely used as fibrous materials for marine industries, since the density of the polyethylene is less than 1. However, the strength of polyethylene is remarkably inferior to those of other synthetic fibrous materials such as polyesters, polyamides and the like. For instance, in the case of ropes, the strength of the ropes made of high-density polyethylene is at most approximately 70% of that of polyester ropes having the same diameter and is at most approximately 50% of that of nylon ropes having the same diameter. For this reason, use of the polyethylene is limited in products, such as towing ropes for large oil tankers, in which high strength is required.